RFR: 8322043: HeapDumper should use parallel dump by default [v4]

[Alex Menkov]

> The fix makes VM heap dumping parallel by default.
> `jcmd GC.heap_dump` and `jmap -dump` had parallel dumping by default, the fix affects `HotSpotDiagnosticMXBean.dumpHeap()`, `-XX:+HeapDumpBeforeFullGC`, `-XX:+HeapDumpAfterFullGC` and `-XX:+HeapDumpOnOutOfMemoryError`.
> 
> Testing:
>   - manually tested different heap dump scenarios with `-Xlog:heapdump`;
>   - tier1,tier2,hs-tier5-svc;
>   - all reg.tests that use heap dump.

Alex Menkov has updated the pull request incrementally with one additional commit since the last revision:

  feedback

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Changes:
  - all: https://git.openjdk.org/jdk/pull/18748/files
  - new: https://git.openjdk.org/jdk/pull/18748/files/f6db604f..482d9ffe

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=jdk&pr=18748&range=03
 - incr: https://webrevs.openjdk.org/?repo=jdk&pr=18748&range=02-03

  Stats: 18 lines in 3 files changed: 0 ins; 3 del; 15 mod
  Patch: https://git.openjdk.org/jdk/pull/18748.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18748/head:pull/18748

PR: https://git.openjdk.org/jdk/pull/18748